In a 9x13-inch pan, place 12 ice cream sandwiches in a single layer. Work quickly here; if the sandwiches sit out for more than 2-3 minutes, they soften and the layers will not set cleanly.
24 ice cream sandwiches
Scoop half of the Cool Whip onto the ice cream layer and spread it evenly using an offset spatula. Make sure the Cool Whip is fully thawed (I pull mine from the freezer the night before). Cold Cool Whip does not spread; it tears the sandwich layer underneath.
1 16 ounces non-dairy whipped topping
Drizzle hot fudge sauce (you may need to warm it just enough to be able to drizzle, but it should not be hot or it will melt the Cool Whip) and caramel sauce over the Cool Whip.
Hot fudge sauce, Caramel sauce
Add the remaining 12 ice cream sandwiches in a single layer, then drizzle on more hot fudge and caramel sauce.
Scoop the rest of the Cool Whip on top of the cake and spread out evenly.
Cover the cake tightly with plastic wrap and freeze for at least 3 hours until completely firm. I tested this at 2 hours and the layers were too soft to slice cleanly. At 3+ hours you get clean slices that hold their shape.
Drizzle more hot fudge and caramel sauce over top of the cake, then sprinkle with M&M’S.
M&M’S for topping
Cut the cake and serve frozen.